With electromechanical wedge brakes, the friction lining is guided by an electrically controlled actuator which moves it at an incline along a wedge surface against the brake object, usually designed as a brake disk. Several variants of a self-boosting electromechanical wedge brake are known.
The basic components of a wedge brake generally consist of a passive wedge plate, an active wedge plate and an actuator to move the active wedge plate relative to the brake object. The active wedge plate usually serves as a support and guide for the friction lining, so that the friction lining can be moved to and from the object to be braked by the movement of the active wedge plate. To reduce the friction between the active and passive wedge plates, a roller bearing or similar can be arranged between the opposing wedge surfaces.
For this reason, wedge brakes basically have a support for the friction lining, which runs at an incline relative to the direction of running of the brake object. For this purpose, there are usually one or more wedge surfaces on the support, along which the guidance of the friction lining takes place. When the friction lining comes into contact with the brake object, it is carried by the brake object in its direction of travel. If the slope of the wedge surface is in the direction of travel of the brake object, the carrying of the friction lining leads to a further advance of the friction lining against the brake object. This intensifies the pressure of the brake lining on the brake object, causing an increase in the braking friction force to be achieved. The braking force thus increases automatically due to the motive energy applied to the brake object, without the friction element having to be actively displaced further against the object by the actuator. This effect is generally known as self boosting. A mathematical treatment of the self-boosting effect is given in Patent EP 0 953 785. According to this, the force FE to be applied by the actuator to the wedge arrangement in order to achieve a specific braking force FB, i.e. the friction force generated on the friction element, reduces according to the following equation:FE=−FB·[1−(tan α/μ)].  (1)
In equation (1), α is the angle between the wedge surface and the movement plane and μ is the coefficient of friction for the material pair consisting of the friction lining and surface of the brake object.
During a switch of the direction of travel of the brake object, the slope of the wedge surface points away from the direction of travel of the brake object so that the pressure of the friction lining on the brake object is reduced. An attenuation of the force exerted by the actuator takes place. The resulting maximum braking force is therefore less than for a movement of the brake object in the direction of the slope of the wedge surface.
In order to be also able to achieve a maximum braking effect with this opposite direction of travel of the brake object, electromechanical wedge brakes have one or more further wedge surfaces whose slope advances the active wedge plate to the brake object during a movement in this opposite direction.
The wedge surfaces of the active wedge plate are usually arranged on the side surface of the wedge plate opposite to the friction lining. The displacement of the active wedge plate by the actuator is effected by a transmission element which connects the active wedge plate to the actuator. The passive wedge plate is, in contrast, fixed relative to the actuator, for example to a brake caliper of the electromechanical wedge brake. Their wedge surfaces are arranged opposite the wedge surfaces of the active wedge plate.
At higher vehicle speeds, the brakes of a vehicle must be able to generate the maximum possible braking force. When electromechanical wedge brakes are used this means that the wedge plates are in the operating position for self-boosting during braking, i.e. that the active wedge plate is displaced in the direction of travel of the brake disk. The maximum braking force is achieved when all the wedge brakes of the vehicle are positioned in the direction of self-boosting for the particular driving direction, i.e. positioned in the same direction.
With a stationery vehicle, it must on the other hand be ensured that the vehicle cannot roll in any of the two possible directions of movement. For this purpose, the wedge brakes of the vehicle are positioned in inverse direction, i.e. a subgroup of the brakes of the brake system of the vehicle is switched to an operating mode with self-boosting in a forward direction of the vehicle and another subgroup of the brakes is switched to an operating position with self-boosting in the rearward direction of the vehicle. Movement of the vehicle due to unevenness or sloping of the roadway is therefore precluded because the braking force for a subgroup of the brakes is always self-boosted and therefore able to hold the vehicle when at a standstill.
Problems do, however, occur during the transition from movement of a vehicle to standstill. If, for example, a vehicle is braked in order to bring it to a standstill within a certain braking distance it is possible that after the vehicle has come to a standstill on a slope on the roadway against the original direction of travel it can roll back because all the wedge brakes are aligned for self-boosting in the original direction of travel and the opposite direction of travel applies only a slight braking force. Therefore, the wedge brakes of the vehicle are positioned in the inverse direction before it comes to a standstill, i.e. one subgroup of the brakes is positioned in the direction of self-boosting opposite to the current direction of travel.
During a switch of a wedge brake from the self-boosting direction to the self-attenuating direction, the active wedge plate moves through the neutral position at which the friction lining no longer contacts the brake object. The braking force in the neutral position is therefore equal to zero. Only when there is a further contact with the brake object does the friction lining again generate a braking force, which due to the prevailing self-attenuating effect can be less than previously generated using the self-boosting effect. As a result, the braking force generated by a brake during the switch from the operating position which is self-boosting to the operating position without self-boosting changes. This variation in the braking force is perceived by the occupants of the vehicle as a jolt which impairs driving comfort.
In the reverse case, during a slow or only partial release of the wedge brakes of a vehicle standing on an incline the wedge brakes positioned against the direction of rolling must be switched to the direction of self-boosting for the rolling direction before the vehicle reaches a specific threshold speed. Here too, a temporary decrease in the braking force occurs, which can be perceived in the interior of the vehicle as a jolt.